Session E13: Minisymposium on Nuclear Physics Deep Underground III

A Large Underground Xenon (LUX) WIMP dark matter experiment

The LUX collaboration has proposed to build a few hundred kilogram dual phase Xenon WIMP dark matter detector at the 4850 foot level of the new SUSEL site in South Dakota. The design builds on the apparent scalability of the recent Xenon10 and Zeplin II experiments, which employed a similar technique. LUX will have an order of magnitude larger volume and introduce a new way to reduce the backgrounds from WIMP like nuclear recoil events, placing the whole detector inside an active water shield/neutron detector. I will briefly describe the design of the detector/shield concept, and summarize the implications for our sensitivity to WIMP dark matter.   

The LUX Apparatus

The LUX experiment will search for Weakly Interacting Massive Particles (WIMPs) using a liquid Xenon time projection chamber. Simultaneous measurement of ionization and scintillation allows for 3D position reconstruction, with a nuclear recoil energy threshold as low as 4.5keV.~ The ratio of ionization and scintillation allows event by event discrimination between nuclear and electronic recoils, and self-shielding provides additional gamma ray background reduction.~ A water shield will protect the detector from gamma rays and neutrons from the surrounding rock. With 300 kg of LXe, LUX is expected to improve sensitivity to WIMPs by a factor of 100. LUX plans to deploy to the Sanford Laboratory at the Homestake mine in late 2008 to begin taking data.~ I describe the construction and strategy behind the LUX detector.   

Background simulations of the XENON100 dark matter detector

The XENON100 detector is a dual-phase xenon time projection chamber (XeTPC) used to search for dark matter in the form of weakly interacting massive particles (WIMPs) by measuring simultaneously the scintillation and ionization signals produced by nuclear recoils. The 65 kg XeTPC is instrumented by 178 PMTs and is surrounded by a 85 kg LXe active veto with 64 PMTs. All materials and components used to build the detector (PMTs, PMT bases, stainless steel, PTFE, copper, etc) have been screened with high purity germanium detectors operating at the Gran Sasso underground laboratory. Special attention has been paid to the choice of construction materials. Using the measured radioactivity as input to the Monte Carlo, we have simulated the response of the XENON100 detector to obtain the expected gamma and neutron backgrounds, which largely determine the sensivity reach of the experiment.   

Scintillation Efficiency of Liquid Xenon for Low Energy Nuclear Recoils

In early 2006, the XENON and ZEPLIN collaborations announced highly stringent upper limits on the WIMP-nucleon cross-section. However, the dominant systematic uncertainty in these limits is due to the uncertainty in the nuclear recoil scintillation efficiency (NRSE) for liquid xenon. The NRSE is defined as the amount of scintillation produced by nuclear recoils, divided by the amount of scintillation produced by electron recoils, per unit energy. ~Though the NRSE has been measured by several groups, its value at the low energies most important for the liquid xenon WIMP searches has a large uncertainty. Furthermore, the NRSE may vary with the strength of the electric field in the liquid xenon. In an attempt to reduce these uncertainties, we have measured the NRSE down to 5 keV nuclear recoil energy for various electric fields.   

The Mini-CLEAN dark matter experiment

The design and current status of the Mini-CLEAN experiment are presented. Mini- CLEAN is an experiment designed to search for nuclear recoils produced by elastic scattering of dark matter particles in a 360 kg noble liquid target. The apparatus may be operated with either liquid argon or liquid neon as the detection material, providing different responses to signal and background. Ionizing radiation events in the noble liquid produce intense scintillation light, which is captured in a spherical array of photomultipliers immersed in the cryogen. Reduction of beta and gamma backgrounds is accomplished through pulse-shape discrimination, which has been shown to be highly effective in both liquid argon and liquid neon. Mini-CLEAN will be installed in SNOLAB in late 2008.   

Depletion of $^{39}$Ar for Direct Dark Matter Detection Experiments

Liquid argon, which has shown excellent background discrimination capabilities, is very suitable for construction of tonne-scale target mass detectors at reasonable cost for the WIMP searches. We have investigated via simulations the pulse shape discrimination (PSD) power and found that it depends strongly on the deposited energy and the detected number of photoelectrons per unit energy. To discriminate the backgrounds from $^{39}$Ar decays for a tonne-scale dark matter detector, it requires a PSD capability better than $10^{10}$, which can only be achievable at a higher threshold energy. Furthermore, without $^{39}$Ar depletion, data acquisition dead-time would be unlikely manageable for a tonne-scale detector and a large scale computing facility would be required to perform on-line data reduction. While with depletion of $^{39}$Ar by a large factor we can not only reduce the background rate, but also make it possible to lower the detection threshold so as to access larger parameter space of WIMPs predicted by minimal super-symmetric models. In this talk, we will further outline the several $^{39}Ar$ depletion technologies under investigation in our institutions.   

Discovery of underground argon with low level of radioactive 39Ar and possible applications to WIMP dark matter detectors.

We report on the first measurement of 39Ar in argon from underground natural gas reservoirs. The gas stored in the US National Helium Reserve was found to contain a low level of 39Ar. The ratio of 39Ar to stable argon was found to be $<$4x10$^{-17}$ (84{\%} C.L.), less than 5{\%} the value in atmospheric argon (39Ar/Ar=8x10$^{-16}$). The total quantity of argon currently stored in the National Helium Reserve is estimated at 1000 tons. 39Ar represents one of the most important backgrounds in argon detectors for WIMP dark matter searches. The findings reported demonstrate the possibility of constructing large multi-ton argon detectors with low radioactivity suitable for WIMP dark matter searches.   

Depleted Argon from Old-Water Underground at Wall, South Dakota

The purpose of this project is to investigate the possibility of using underground water as a source for depleted argon which will be the target material for next generation dark matter detectors at deep underground laboratories and to design a machine that would extract argon from underground water. The only source of $^{39}$Ar from old underground water is $^{39}$Ar that is produced from (n,p) reactions with $^{39}$K. An analysis of the soil was conducted to determine the $^{39}$K content and the number of free neutrons due to ($\alpha $,n) reactions induced by $^{232}$Th and $^{238}$U decay. This was done with atomic absorption spectrometry and a low background counting facility, respectively. The results indicated that the soil contains approximately 2{\%} $^{39}$K and 2 neutrons/y/g/ppm. As a result, $^{39}$Ar is predicted to be about a factor of 70 lower than atmospheric level. In addition, a machine was designed that would be capable of extracting argon from underground water.   

Progress Towards A 60 kg Bubble Chamber for Dark Matter Detection

The COUPP collaboration~has been investigating the use of bubble chambers for direct detection of dark matter. Our progress in developing a stable 2 kg CF3I bubble chamber with long superheated life time has led us to start construction on a significantly larger detector with a sensitive mass of 60 kg. I will give an overview of the challenges in engineering this detector and discuss the status of its construction.